If the hair loss is small patches, is auto immune system being affected due to over treatment also being something that can happen.  Extract from that page:

Alopecia Areata

Alopecia areata results in the appearance of roughly circular bald patches on the scalp. The skin in the affected area is smooth and soft and has no hair at all. 

Unlike androgenetic alopecia, alopecia areata often generates an abrupt hair loss. It is now known to be an auto-immune disorder, which causes hair follicles to stop producing hair, and there is commonly a hereditary factor. Research has shown that in alopecai areata the hair follicles in the anagen growing phase become a target for attack by auto-immune cells. Hair loss is sudden, sometimes even overnight.

Alopecia areata may be no more than a few bald patches that appear and then regrow hair. If the patches are small, the chances of regrowth are very good.

Diffuse alopecia areata leads to sudden thinning without the bald patches. This condition is far less common than the bald patches described above. Alopecia Totalis is the term used when all the hair on the head is affected, and when all the body hair is affected too, that will be called alopecia Universalis. When this happens, the chances of regrowth are poor.